According to different functional uses, semiconductor memories can be classified as Random Access Memories (RAMs) (also referred to as read-write memory) and Read Only Memories (ROMs). And RAMs can be further classified as Static Random Access Memories (SRAMs) and Dynamic Random Access Memories (DRAMs).
As is known to all, data is written into a ROM by a specialized device and will not vanish when the ROM is powered off. In other words, the ROM is non-volatile. However, once data is written into a ROM, it cannot be optionally rewritten anymore. A SRAM can be accessed at a high speed. But after the SRAM is powered off, data in the SRAM will be lost.
Due to the characteristics of ROMs and SRAMs mentioned above, in a field of data processing, ROMs and SRAMs are both applied in many kinds of devices, such as personal digital assistants, tablet personal computers, and so on. Nevertheless, ROMs and SRAMs take up a very large amount of space, which makes it difficult to make data processing devices, such as handheld devices, become smaller and smaller as people are expecting.